The present invention relates in general to communication systems and components therefor, and is particularly directed to an integrated optical amplifier architecture containing a plurality of spatially adjacent optical fibers, or channeled waveguide amplifier channels in an integrated assembly. The waveguide amplifier channels are aligned at their respective inputs and outputs to the optical fibers of a multi-fiber ribbon, and are optically and mechanically integrated with a multi-channel optical interface that focuses optical pumping energy into the waveguide amplifier channels from a direction that is generally transverse to the substrate.
Because of bundle density limitations associated with the individual buffered fibers and connector interface configurations of legacy, single mode optical fiber cables (a reduced complexity cross-section of one of which is shown diagrammatically at 10 in FIG. 1), especially those containing a relatively large number of fiber strands, optical communication equipment and component suppliers have begun offering relatively thin, multiple optical fiber-containing ribbons and small form factor multi-channel connector interfaces.
As diagrammatically illustrated in FIG. 2, the flat, rectangular cross-section of a multi-fiber ribbon 20 facilitates densely packing a relatively large number of such fibers 21 within a physical volume that is both compact and readily conformal with a variety of housing and equipment surfaces.
Unfortunately, when employed in applications requiring amplification of the optical signals being transported by the fibers, such as in long haul repeaters, it is necessary to break out each individual fiber 21 from the ribbon 20, as diagrammatically illustrated in FIG. 3, and then connect each fiber to its own dedicated optical amplifier unit. Such an optical amplifier unit, a component block diagram of which is shown in FIG. 4 and a diagrammatic optical fiber signal transport view of which is shown in FIG. 5, is typically a relatively large sized and costly piece of equipment.
These size and cost drawbacks are due to the number of individual fiber-interfaced components employed, the use of relatively long loops 31 of optical pumping energy absorbing and amplifying material (such as erbium-doped fiber) for gain, the need for relatively narrow spectrum, distributed feedback laser diode pumps 32 (which require thermoelectric coolers and associated control circuits therefor), as well as the hand labor intensity involved in physically interfacing individual components and the input and output ports 33, 34 of each amplifier unit with a respective fiber of the ribbon fiber bundle.
In accordance with the present invention, these and other shortcomings of conventional, individual fiber-dedicated light amplifiers are effectively obviated by a new and improved, multi-fiber ribbon-interfaced optical amplifier architecture, having a very compact form factor that facilitates one-for-one alignment with and coupling to each of the optical fibers of a multi-fiber ribbon, and containing a physically compact, optical focusing structure that directs the optical outputs from multiple pumping energy sources into the optical fiber or channeled waveguide amplifying channels to which the ribbon fibers are coupled.
As will be described, the amplifier comprises a generally planar support substrate of bulk material such as a glass, containing a plurality of spatially adjacent (e.g., parallel) optical waveguide channels or a collection of individual amplifying fibers in a precision support substrate, such as, but not limited to a V-grooved silicon substrate, that are aligned with the signal transport ribbon fibers. The relatively narrow dimensions of various components of the multi-channel fiber optic amplifier of the invention are readily obtained using industry standard semiconductor mask-etch processing techniques. Each optical waveguide channel may include a central core through which a signal light beam supplied by a respective fiber propagates, and a surrounding cladding layer.
The core is dimensioned to nominally conform with an associated ribbon fiber and serves as the principal signal beam transport and amplifying medium of the channel. The core may comprise an optically transmissive material whose photonically stimulated, energy state transfer properties readily absorb optical energy supplied by one or more light amplification pumping sources and provides emitted radiation-stimulated amplification of the signal beam. As a non-limiting example, the optical waveguide core may comprise erbium ytterbium-doped phosphate glass.
The core may be double clad with an inner cladding comprised of the same phosphate glass with a slightly lower index of refraction than the core and is employed to increase the focusing tolerance window upon which pumping optical energy is imaged, and increase pumping power available along the gain length of the amplifier, as the pump energy propagates downstream in the low loss cladding waveguide. The surrounding cladding layer is not necessary where the pumping source and imaging optics can achieve very narrow single mode imaging tolerances. For accurate alignment of the optical waveguide channels with the multi-channel pump source and ribbon fibers, the substrate may be patterned to include a plurality of spatially adjacent (e.g., parallel) xe2x80x98Vxe2x80x99-shaped linear grooves, into which respective clad core-configured fiber waveguide channels are affixed.
Pumping energy is coupled to the waveguide channels by means of a multi-channel optical interface immediately adjacent to the substrate surface and configured to focus pumping energy from a plurality of spatially adjacent pumping energy sources into the waveguide channels. As a non-limiting example, the pumping energy sources may comprise a one-dimensional (1xc3x97N) array of diode-laser emitter elements, such as edge-emitting laser diodes, vertical cavity surface emitting laser (VCSEL) elements, and the like. The use of an MXN array of pumping elements increases the power density per channel and provides redundancy for each channel.
In accordance with a first embodiment, the outputs from the array of diode-laser sources are focussed into the waveguide channels by a corresponding MXN array of micro-lenses or diffractive optic elements (DOES) distributed over a pumping beam-receiving surface of a refracting prism. The micro-lenses and the geometry and refractive index of the prism are defined such that each focussed pumping beam is coupled by the prism into a respective optical waveguide channel so that the pumping energy is confined to the waveguide channel, and undergoes multiple reflections as it repeatedly passes back and forth between the cladding layer and the signal-transporting core, where the pump energy is absorbed during propagation along the channel.
In respective second and third embodiments, the prism and focusing lens array of the first embodiment may be replaced by an array of gradient indexed (GRIN) lenses or a two-dimensional (e.g., spherical) lenslet array. The number of GRIN lenses or lenslets corresponds to the number of pumping source elements, to provide one-for-one collimation or focusing of the light beams generated by the pumping energy emitters into the optical waveguide channels.